When you do a keyword search in a database, you are searching many different fields for the words you typed in. Your keywords could show up in the title, the author, source, or the abstract of any article. You get a lot of hits, but your results are not very precise.
When you do a subject search in a database, you are searching one field--the subject field--for the word you typed in. You don't get as many hits, but your results are precise because the subject field has words that describe the main topic of the article.
Think of it this way:
When you do a keyword search, it's like looking around in a hardware store. You will find lots of interesting and useful stuff, but you might not find exactly what you are looking for.
When you do a subject search, it's like asking the store manager to show you where the nails are.
When reviewing the literature of your field for your thesis, do both keyword and subject searches.
A basic keyword search follows this syntax:
This search retrieved 49 results in EBSCOhost.
You can narrow down the number of results you get by using more keywords:
cats scratching allergy
This search retrieved 2 results in EBSCOhost
You can expand the number of results you get by using synonyms and truncation:
(cats or kittens) scratch*
This search retrieved 548 results in EBSCOhost.
The reason a subject search is so precise is that the professionals who index articles for databases assign each article several specific words that describe the main ideas in the article.
These words are located in the subject field of the article's record. The indexers use a specific vocabulary to choose the words they assign to each article. This specific vocabulary is listed in a thesaurus.
Different disciplines have different thesauri because different disciplines use different words to describe their unique disciplinary knowledge.